Flat-panel organic LED display devices employ a variety of technologies for emitting patterned, colored light to form full-color pixels. In one approach, different materials emitting light of different colors in response to a current are patterned over a surface. However, such patterning is difficult and expensive and current technologies, such as the use of shadow-masks, are limited in resolution and size. In an alternative approach, a common light-emitter is employed for all of the pixels. One such design relies upon a white-light emitting common organic layer in combination with color filters, for example red, green, and blue, to create a full-color display. The color filters may be located on the substrate, for a bottom-emitter, or on the cover, for a top-emitter. For example, U.S. Pat. No. 6,392,340 entitled “Color Display Apparatus having Electroluminescence Elements” issued May 21, 2002 illustrates such a device. However, such designs are relatively inefficient since approximately two-thirds of the light emitted may be absorbed by the color filters.
In another design, color-change materials convert light, for example blue light, of the common light-emitter into different colored light of the desired frequencies. The color-change materials absorb the high-frequency light and re-emit light at lower frequencies. For example, an OLED device may emit blue light suitable for a blue sub-pixel and employ a green color-change material to absorb blue light to emit green light and employ a red color change material to absorb blue light to emit red light. The color-change materials may be combined with color filters to further improve the color of the emitted light and to absorb ambient light and avoid exciting the color-change materials with ambient light, thereby improving device contrast. US20050116621 A1 entitled “Electroluminescent devices and methods of making electroluminescent devices including a color conversion element” describes the use of color-change materials or (color-conversion elements).
U.S. Patent Application 20040233139A1 discloses a color conversion member which is improved in the prevention of a deterioration in color conversion function, the prevention of reflection of external light, and color rendering properties. The color conversion member comprises a transparent substrate, two or more types of color conversion layers, and a color filter layer. The color conversion layers function to convert incident lights for respective sub-pixels to outgoing lights of colors different from the incident lights. The two or more types of color conversion layers are arranged on said transparent substrate. The color filter layer is provided on the transparent substrate side of any one of the color conversion layers or between the above any one of the color conversion layers and the color conversion layers adjacent to the above any one the color conversion layers. US 20050057177 also describes the use of color change materials in combination with color filters.
In general, color-change material systems suffer from efficiency problems. The production of relatively higher-frequency blue light can be difficult and the conversion of light from relatively higher frequencies to relatively lower frequencies may not be efficient or the conversion materials may fade over time, reducing the performance of the display. Moreover, much of the relatively higher-frequency light may not interact with the color-change materials and thus may not be converted to the desired, relatively lower frequency light. U.S. 2005/0140275A1 describes the use of red, green, and blue conversion layers for converting white light into three primary color of red, green, and blue light. However, the efficiency of emitted-light conversion remains a problem.
OLEDs rely upon thin-film layers of organic materials coated upon a substrate. OLED devices generally can have two formats known as small molecule devices such as disclosed in U.S. Pat. No. 4,476,292 and polymer OLED devices such as disclosed in U.S. Pat. No. 5,247,190. Either type of OLED device may include, in sequence, an anode, an organic EL element, and a cathode. The organic EL element disposed between the anode and the cathode commonly includes an organic hole-transporting layer (HTL), an emissive layer (EL) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the EL layer. Tang et al. (Appl. Phys. Lett., 51, 913 (1987), Journal of Applied Physics, 65, 3610 (1989), and U.S. Pat. No. 4,769,292) demonstrated highly efficient OLEDs using such a layer structure. Since then, numerous OLEDs with alternative layer structures, including polymeric materials, have been disclosed and device performance has been improved.
Light is generated in an OLED device when electrons and holes that are injected from the cathode and anode, respectively, flow through the electron transport layer and the hole transport layer and recombine in the emissive layer. Many factors determine the efficiency of this light generating process. For example, the selection of anode and cathode materials can determine how efficiently the electrons and holes are injected into the device; the selection of ETL and HTL can determine how efficiently the electrons and holes are transported in the device, and the selection of EL can determine how efficiently the electrons and holes be recombined and result in the emission of light, etc.
It has also been found, that one of the key factors that limits the efficiency of OLED devices is the inefficiency in extracting the photons generated by the electron-hole recombination out of the OLED devices. Due to the high optical indices of the organic materials used, most of the photons generated by the recombination process are actually trapped in the devices due to total internal reflection. These trapped photons never leave the OLED devices and make no contribution to the light output from these devices. Because light is emitted in all directions from the internal layers of the OLED, some of the light is emitted directly from the device, and some is emitted into the device and is either reflected back out or is absorbed, and some of the light is emitted laterally and trapped and absorbed by the various layers comprising the device. In general, up to 80% of the light may be lost in this manner.
A typical OLED device uses a glass substrate, a transparent conducting anode such as indium-tin-oxide (ITO), a stack of organic layers, and a reflective cathode layer. Light generated from the device is emitted through the glass substrate. This is commonly referred to as a bottom-emitting device. Alternatively, a device can include a substrate, a reflective anode, a stack of organic layers, and a top transparent cathode layer. Light generated from the device is emitted through the top transparent electrode. This is commonly referred to as a top-emitting device. In these typical devices, the index of the ITO layer, the organic layers, and the glass is about 2.0, 1.7, and 1.5 respectively. It has been estimated that nearly 60% of the generated light is trapped by internal reflection in the ITO/organic EL element, 20% is trapped in the glass substrate, and only about 20% of the generated light is actually emitted from the device and performs useful functions.
A variety of techniques have been proposed to improve the out-coupling of light from thin-film light emitting devices. Such techniques include the use of diffraction gratings, brightness enhancement films having diffractive properties, reflective structures, and surface and volume diffusers. The use of micro-cavity techniques is also known. However, none of these approaches cause all, or nearly all, of the light produced to be emitted from the device. Moreover, diffractive techniques cause a significant frequency dependence on the angle of emission so that the color of the light emitted from the device changes with the viewer's perspective. Scattering techniques are also known and described in, for example, co-pending, commonly assigned U.S. Ser. No. 11/065,082, filed Feb. 24, 2005 entitled “OLED device having improved light output” by Cok which is hereby incorporated in its entirety by reference.
It is also known to combine layers having color-conversion materials with scattering particles to enhance the performance of the color-conversion materials by increasing the likelihood that incident light will interact with the color-conversion materials, thereby reducing the concentration or thickness of the layer. Such combination may also prevent light emitted by the color-conversion material from being trapped in the color-conversion material layer. US20050275615 A1 entitled “Display device using vertical cavity laser arrays” describes such a layer as does US20040252933 entitled “Light Distribution Apparatus”. US20050012076 entitled “Fluorescent member, and illumination device and display device including the same” teaches the use of color-conversion materials as scattering particles. US20040212296 teaches the use of scattering particles in a color-conversion material layer to avoid trapping the frequency-converted light. Co-pending, commonly assigned U.S. Ser. No. 11/361,094, filed Feb. 24, 2006 entitled “Light-Scattering Color-Conversion Material Layer” by Cok which is hereby incorporated in its entirety by reference describes integral light-scattering color-conversion material layers.
However, as described in U.S. Ser. No. 11/065,082, by Cok referenced above, the use of scattering layers without the use of low-index layers results in reduced sharpness in a pixilated device. Moreover, low-index layers, particularly in bottom-emitter devices, are difficult to build. Co-pending, commonly assigned U.S. Ser. No. 11/387,192, filed Mar. 23, 2006 entitled “OLED Device having Improved Light Output” by Cok which is hereby incorporated in its entirety by reference describes such a bottom-emitter OLED device, but the formation of the low-index elements described therein may be difficult. Hence, the use of scattering layers in concert with color-change materials is problematic, particularly for bottom-emitter OLED devices. In the absence of such scattering layers, prior-art solutions incorporating color-change materials for extracting trapped light may not be effective.
There is a need therefore for an improved color-change material layer that may be employed in organic or inorganic light-emitting diode device structures, that improves the efficiency and sharpness of the device.